1. Field of the Invention
The present invention relates to a switching power source apparatus that is suitably used in a computer display monitor or a large-sized television image receiver in which a high resolution video signal is displayed using, for example, a cathode ray tube. More particularly, the invention is intended, in a display monitor or television image receiver, which uses, for example, a cathode ray tube, to enable the generation of a plurality of voltages including the generation of a high voltage efficiently.
2. Description of the Related Art
As a power source apparatus used in a computer display monitor, etc. for making display of a video signal having a high resolution, there has hitherto been used an apparatus such as that illustrated in FIG. 9. This apparatus uses an insulated type switching power source circuit, a horizontal deflection circuit, and a high-voltage generation circuit.
Namely, in FIG. 9, a commercially available alternating power source (AC) 100 is connected to a smoothing capacitor 102 through a diode bridge rectification circuit 101. A negative-polarity end of the capacitor 102 is grounded and a positive-polarity end thereof is connected to an oscillation drive circuit 104 through a resistor 103. This positive-polarity end of the capacitor 102 is grounded through a switching circuit part 105 comprised of a serial circuit of switching elements Qa1 and Qa2. And, these switching elements Qa1 and Qa2 are driven so that these elements may be alternately made conductive by a prescribed frequency by the oscillation drive circuit 104.
Further, the switching circuit part 105 constitutes a half-bridge circuit; the positive-polarity end of the capacitor 102 is connected to the drain of the switching element Qa1; and the source of the switching element Qa2 is grounded. Also, to the switching elements Qa1 and Qa2 there are respectively connected in parallel damper diodes Da1 and Da2. And, a point of connection between the source of the switching element Qa1 and the drain of the switching element Qa2 is grounded through a resonance capacitor 106, a choke coil 107, and a primary winding La1 of an insulated type converter transformer 108.
As a result of this, into the primary winding La1 of the converter transformer 108 there is made to flow a resonance current that is inverted according to the oscillating frequency of the oscillation drive circuit 104. Thereby, a so-called "separately-excited type" of current-resonance type of converter power source circuit is constructed. Namely, in this circuit, the fundamental operation on a primary side of the converter transformer 108, when typically illustrated, is as illustrated in FIGS. 10A to 10C. In these FIGS. 10A to 10C, an equivalent circuit that is prepared when the switching element Qa1 has been made "on" by a drive pulse output from the oscillation drive circuit 104 illustrated in FIG. 10A is illustrated in FIG. 10B. And, an equivalent circuit that is prepared when the switching element Qa2 has been made "on" by that drive pulse is illustrated in FIG. 10C.
On this account, when the switching element Qa1 has been made "on", a switch 201 corresponding to the switching element Qa1 in the equivalent circuit in FIG. 10B is closed. Therefore, there is constructed a serial resonance circuit that is comprised of a direct current voltage source 203 corresponding to the positive-polarity end of the capacitor 102, the resonance capacitor 106, an inductor 204 including the choke coil 107 and primary winding La1, and a resistor 205. And, using the direct current voltage source 203 as a power source, a positive-polarity resonance current is made to flow through that circuit by way of the switch 201.
Next, when the switching element Qa2 has been made "on", a switch 202 corresponding to the switching element Qa2 in the equivalent circuit in FIG. 10C is closed. Therefore, by way of this switch 202, a negative-polarity resonance current is made to flow through the serial resonance circuit that is comprised of the resonance capacitor 107, the inductor 204, and the resistor 205. And, in this way, the positive-polarity and negative-polarity resonance currents are alternately generated according to the drive pulses output from the oscillation drive circuit 104, whereby a desired frequency of alternating current is made to flow through the serial resonance circuit.
Further, the waveforms of the currents flowing through respective portions of each of the equivalent circuits illustrated in FIGS. 10A to 10C are illustrated in FIGS. 11A to 11C. Here, FIG. 11A and FIG. 11B illustrate the waveforms of the current IQ1 and current IQ2 flowing, respectively, through the switching elements Qa1 and Qa2 while FIG. 11C illustrates the waveform of a resonance current I1 flowing through the serial resonance circuit. Also, in FIG. 12, illustration is made of the relationship between the resonance current I1 flowing through the serial resonance circuit and the frequency f. In FIG. 12, f0 represents the resonance frequency of the serial resonance circuit of FIGS. 10A to 10C and fsw represents the repetitive operating frequency of the switching circuit part 105 that is driven by the oscillation drive circuit 104.
In this case, assume that C, L, and R represent the values of the resonance capacitor 106, inductor 204, and resistor 205, respectively, and that Z represents the impedance of the serial resonance circuit with respect to each frequency .omega.. Determining the admittance under this assumption, the admittance Y is expressed in the form of the following [equation no. 1]. ##EQU1##
On the other hand, the resonance frequency f0 of the serial resonance circuit is expressed in the form of the following [equation no. 2]. ##EQU2##
Here, because the current I is in proportion to the Y of the [equation no. 1], when showing the magnitude of the current I1 as measured with respect to the frequency by the use of that Y, the variation thereof is as indicated by a curve of FIG. 12. The resonance current has a maximum value at the resonance frequency f0. Also, the repetitive operating frequency fsw of the switching circuit part 105 comprised of the switching elements Qa1 and Qa2 is so set as to move along the right side of this resonance current curve, i.e., so that, in these frequencies, the relationship of fsw&gt;f0 may be satisfied.
On this account, standing on the above-described fundamental operation, the entire circuit operation of FIG. 9 will now be explained in detail. The switching operation of the power source circuit in the circuit construction of this FIG. 9 is performed as follows. First, using as the charging current the rectification current that is obtained by rectifying the commercially available alternating power source 100, which is closed, with the diode bridge rectification circuit 101, a rectified and smoothed voltage is generated across the both ends of the smoothing capacitor 102. Further, using this rectified and smoothed voltage as the operating power source, a power source is supplied to the oscillation drive circuit 104 through the resistor 103. And the drive pulses that are alternately generated in the oscillation drive circuit 104 are supplied to the switching elements Qa1 and Qa2, respectively.
And, with certain timing, from the oscillation drive circuit 104, for example, a positive drive pulse is supplied to the switching element Qa1 and, conversely, a negative drive pulse is supplied to the other switching element Qa2 constituting the switching circuit part 105. As a result of this, the switching element Qa1 becomes "on" and the switching element Qa2 becomes "off". Then, a positive-polarity resonance current is supplied to the serial resonance circuit comprised of the resonance capacitor 106, choke coil 107, and primary winding La1 of the insulated type converter transformer 108 via the switching element Qa1.
Further, with the next timing, from the oscillation drive circuit 104, for example, a negative drive pulse is supplied to the switching element Qa1 and, conversely, a positive drive pulse is supplied to the other switching element Qa2 constituting the switching circuit part 105. As a result of this, the switching element Qa1 immediately becomes "off" and the switching element Qa2 becomes "on". Then, a negative-polarity resonance current is supplied to the serial resonance circuit comprised of the resonance capacitor 106, choke coil 107, and primary winding La1 of the insulated type converter transformer 108 via the switching element Qa2.
By the serial resonance current obtained by this operation being repeatedly performed, the converter transformer 108 is excited. And, alternating output voltages are taken out from secondary windings La2, La3, La4, and La5 that have been made on a secondary side of the converter transformer 108. Further, to these secondary windings La2, La3, La4, and La5, there are connected rectification circuits (diodes) 109, 110, 111, and 112 and smoothing circuits (capacitors) 113, 114, 115, and 116 for taking out the direct current voltages from those alternating output voltages, respectively.
In this way, from the secondary windings La2, La3, La4, and La5 of the converter transformer 108, through the rectification circuits 109, 110, 111, and 112 and smoothing circuits 113, 114, 115, and 116, there are taken out a so-called "+B voltage" (the voltage value of that is E0) that becomes a power source voltage of, for example, a horizontal deflection circuit or a high-voltage generation circuit and the other voltages (the voltage values of that are E2, E3, and E4) that are used as the power source voltages of respective signal-operation circuits.
Also, the conversion to constant voltage of the so-called "+B voltage" that is obtained from the secondary winding La2 of the converter transformer 108 and that becomes a power source voltage of the horizontal deflection circuit or high-voltage generation circuit is performed, for example, as follows. Namely, assume, for example, that the brightness of an image displayed on the cathode ray tube rises with the result that a high-voltage load fluctuates so as to increase, or that the horizontal amplitude of the image displayed on the cathode ray tube fluctuates so as to become wide. Then, the load with respect to the +B voltage increases. As a result, the voltage value E0 of the +B voltage tends to fluctuate so as to become small.
On that account, this fluctuation of voltage is taken out by a voltage detection part comprised of resistors 117 and 118 and is error-amplified by a control circuit 119. Thereafter, the resulting voltage is sent to the oscillation drive circuit 104 for controlling and driving the frequency of the switching circuit part 105, via the photo-coupler 120 for performing the insulation of the constant-voltage control system. And, the operating frequency of the drive pulse output from the oscillation drive circuit 104 in correspondence with that voltage is so controlled as to become low. As a result, the switching frequency fsw of the switching circuit part 105 is decreased.
Here, in the above-described power source circuit, the switching frequency fsw of the switching circuit part 105 is set higher than the resonance frequency of the serial resonance circuit comprised of the resonance capacitor 106, choke coil 107, and primary winding La1 of the insulated type converter transformer 108. Accordingly, in case that the switching frequency fsw has been so controlled as to become low, this switching frequency fsw approaches to the resonance frequency f0 of the serial resonance circuit in FIG. 12. As a result, the excitation current flowing through the primary winding La1 increases, whereby the conversion to constant voltage is achieved.
Conversely, assume that the brightness of the image displayed on the cathode ray tube decreases with the result that the high-voltage load fluctuates so as to decrease, or that the horizontal amplitude of the image displayed on the cathode ray tube fluctuates so as to become narrow. Then, the voltage value E0 of the +B voltage fluctuates so as to become large. For this reason, in the same way as stated above, the control signal is sent to the oscillation drive circuit 104 through the photo-coupler 120. Thereby, the operating frequency of the drive pulse output from the oscillation drive circuit 104 in correspondence with that resulting voltage is so controlled as to become high. As a result, the switching frequency fsw of the switching circuit part 105 is increased.
Accordingly, in case that the switching frequency fsw has been so controlled as to become high, the switching frequency fsw becomes separated from the resonance frequency f0 of the serial resonance circuit in FIG. 12. As a result, the excitation current flowing through the primary winding La1 of the converter transformer 108 is suppressed, whereby the conversion to constant voltage is achieved. Also, at this time, regarding the other voltages (the voltage values of that are E2, E3, and E4) that are taken out from the secondary windings La3, La4, and La5 of the same converter transformer 108 as well, the conversion to constant voltage is achieved, substantially, by so-called "cross-regulation".
Furthermore, the +B voltage that has been obtained from the secondary winding La2 of the converter transformer 108 in that way is supplied to the horizontal deflection circuit comprised of a horizontal oscillation drive circuit 122, horizontal output circuit 123, and horizontal deflection yoke 124, via, for example, a horizontal amplitude pin-distortion correction circuit 121. Also, the +B voltage that has been obtained from the secondary winding La2 of the converter transformer 108 is also supplied as a power source of the high-voltage generation circuit constructed of a high-voltage oscillation drive circuit 125, switching circuit part 126, control circuit 127, and high-voltage transformer 128.
Next, an explanation will be given of the high-voltage generation circuit. In FIG. 9, the high-voltage generation circuit is constructed of a separately-excited type of current resonance type converter. And, the drain of a switching element Qa3 is connected to the +B voltage and the source of a switching element Qa4 is grounded so that the two switching elements Qa3 and Qa4 constituting the switching circuit part 126 may construct a half-bridge circuit. Also, between the source and the drain of the switching elements Qa3 and Qa4 there are respectively connected damper diodes Da3 and Da4.
Further, to a point of connection between the source of the switching element Qa3 and the drain of the switching element Qa4 there are connected in series a resonance capacitor 129, choke coil 130, and high-voltage transformer 128 such as a flyback transformer (FBT). And, to the switching elements Qa3 and Qa4 there are supplied from the high-voltage oscillation drive circuit 125 rectangular drive pulses that have different polarities and that are intended to alternately make these switching elements "on" and "off" in units of a half period.
Namely, the switching operation of the high-voltage generation circuit that has the above-described construction is performed as follows. First, when a power source is supplied from the +B voltage to the high-voltage oscillation drive circuit 125 by way of the resistor 131 whereby the +B voltage is supplied to this high-voltage generation circuit, a positive drive pulse is supplied from the high-voltage oscillation drive circuit 125 to the switching element Qa3. Resultantly, the switching element Qa3 becomes "on". And, via the switching element Qa3, a positive resonance current is supplied to the resonance capacitor 129 and to a primary winding Lb0 of the high-voltage transformer 128.
Next, a negative drive pulse is supplied to the switching element Qa3 and, conversely to this, a positive drive pulse is supplied to the switching element Qa4. Resultantly, the switching element Qa3 immediately becomes "off" and the switching element Qa4 becomes "on". Resultantly, via the switching element Qa4, a negative resonance current is supplied to the resonance capacitor 129 and to the primary winding Lb0 of the high-voltage transformer 128. By this operation being repeatedly performed, the serial resonance current excites the high-voltage transformer 128. Thereby, alternating output voltages are taken out from high-voltage windings Lbl to Lb9 that have been made on a secondary side of the high-voltage transformer 128.
Further, for performing full-wave rectification with respect to the positive and negative alternating voltages, regarding the windings Lb1 to Lb9, the windings Lb6 to Lb9 and diodes Db6 to Db9 are connected in series to each other. And, the windings Lb1 to Lb4 have diodes Da1 to Db5 respectively connected in series thereto so as to have an opposite polarity to that of the windings Lb6 to Lb9. Thereafter, the both groups of winding are connected to each other. Also, to this portion of connection there is connected the winding Lb5 one end of that is made open. And, equivalently providing a smoothing capacitor, the rectified voltages obtained from the windings Lb1 to Lb4 and those obtained from the windings Lb6 to Lb9 are serially accumulated up. It is thereby arranged that a high-voltage output voltage EHT be obtained through the use of an output capacitor 132.
And, the conversion to constant voltage of the high-voltage output voltage EHT obtained from the high-voltage windings Lb1 to Lb9 is performed in the same way as in the case of the equivalent circuit of FIG. 10 for example as follows. Namely, in this high-voltage generation circuit, under the assumption that f01 represents the resonance frequency of a serial resonance circuit constructed of the serial resonance capacitor 129, choke coil 130, and primary winding Lb0 of the high-voltage transformer 128, the following setting is done beforehand. Namely, the switching frequency fsw1 of a switching circuit constructed of the half-bridge converter is set higher than the resonance frequency f01.
On this account, assuming that the brightness of the image displayed on, for example, a cathode ray tube increases with the result that the high-voltage load fluctuates so as to increase, the high-voltage output voltage EHT fluctuates so as to decrease. This voltage fluctuation is taken out by a voltage detection circuit constructed of resistors 133 and 134. And a control signal obtained through the operation of the control circuit 127 is sent to the high-voltage oscillation drive circuit 125. Thereby, the operating frequency of the drive pulse that is output from the high-voltage oscillation drive circuit 125 in correspondence with that resulting voltage is so controlled as to become low. As a result, assuming that fsw1 represents the switching frequency of the switching elements Qa3 and Qa4, this switching frequency fsw1 decreases.
Conversely, assuming that the brightness of the image displayed on the cathode ray tube decreases with the result that the high-voltage load fluctuates so as to decrease, the high-voltage output voltage EHT fluctuates so as to increase. This voltage fluctuation is taken out by the voltage detection circuit constructed of the resistors 133 and 134. And a control signal obtained through the operation of the control circuit 127 is sent to the oscillation drive circuit 125. Thereby, the operating frequency of the drive pulse that is output from the high-voltage oscillation drive circuit 125 in correspondence with that resulting voltage is so controlled as to become high. As a result, the switching frequency fsw1 of the switching elements Qa3 and Qa4 increases.
Accordingly, in the setting of the previously stated high-voltage generation circuit, when the brightness of the image displayed on the cathode ray tube increases and in consequence the high-voltage load increases, the high-voltage output voltage EHT fluctuates so as to become low. For this reason, the switching frequency fsw1 is so controlled as to become low. However, at this time, the switching frequency fsw1 approaches the resonance frequency f01 of the serial resonance circuit. Resultantly, the excitation current flowing through the primary winding Lb0 increases, whereby the conversion to constant voltage is achieved.
Conversely, when the brightness of the image displayed on the cathode ray tube decreases and in consequence the high-voltage load fluctuates so as to decrease, the high-voltage output voltage EHT fluctuates so as to increase. For this reason, the switching frequency fsw1 is so controlled as to become high, with the result that the switching frequency fsw1 goes away from the resonance frequency f01 of the serial resonance circuit. Resultantly, the excitation current flowing through the primary winding Lb0 is suppressed with the result that the conversion to constant voltage is achieved.
Further, the high-voltage transformer 128 has provided therein a secondary winding Lc1 for obtaining a voltage E1 used as the detection voltage for use for a protection circuit in addition to the primary winding Lb0, to which the excitation current is supplied, and the high-voltage windings Lb1 to Lb9 for obtaining the high-level output voltage EHT for supplying an anode voltage to the cathode ray tube, as the secondary winding. And, the alternating output voltage taken out from the secondary winding Lc1 of this high-voltage transformer 128 is supplied to the smoothing capacitor 135 through the rectification diode Dc1. Thereby, the voltage E1, which is used as the detection voltage for use for the protection circuit, is taken out.
Further, in FIG. 13, illustration is made, as a block diagram, of the entire construction of the above-described conventional apparatus that includes the insulated-type switching power source circuit, horizontal deflection circuit, and high-voltage generation circuit. In this FIG. 13, the commercially available alternating power source is rectified in an AC rectification/smoothing circuit 301. Using the rectified current obtained by that rectification as the charging current, a rectified/smoothed voltage is obtained across the ends of the smoothing capacitor. This rectified/smoothed voltage is used as the operating power source. And, a converter circuit 303 is made to perform the switching operation with the use of a drive pulse obtained from an oscillation drive circuit 302. A converter transformer 304 is thereby excited and, from this converter transformer 304, an output voltage is taken out.
Using the thus-taken-out output voltage, a horizontal output circuit 306 is made to perform the switching operation with the use of a drive pulse obtained from the horizontal oscillation drive circuit 305. Thereby, a deflection current is supplied to a horizontal deflection yoke 307. Along with this, an output voltage from the converter transformer 304 is supplied to a control circuit 308, and a control signal from this control circuit 308 is supplied to the oscillation drive circuit 302. By doing so, the output voltage from the converter transformer 304 is stabilized.
Also, using an output voltage that is taken out from the converter transformer 304, a high-voltage output circuit 310 is made to perform the switching operation with the use of a drive pulse obtained from a high-voltage oscillation drive circuit 309. Thereby, a high-voltage transformer 311 is excited to cause the generation of a high voltage from this high-voltage transformer 311. Thereby, a high voltage is supplied to the anode of a cathode ray tube 312. Along with this, the output voltage of the high-voltage transformer 311 is supplied to a control circuit 313, a control signal from that is supplied to the high-voltage oscillation drive circuit 309. Thereby, the output voltage from the high-voltage transformer 311 is stabilized.
In this way, the +B voltage and the high-voltage and further other voltages are formed. However, in the above-described conventional switching power source apparatus, there are problematic points to be improved from the economical point of view as well as from the viewpoint of the effective utilization of the energy resources. Namely, a first problematic point is the loss of electric power in the switching circuit part and a second problematic point is the efficiency of conversion in the switching converter output transformer. These two problematic points will hereafter be explained.
Namely, this switching power source apparatus, as a first problem, has a circuit construction that is equipped with power source circuit parts having the function of providing a plurality of constant-level output voltages. In addition, the circuit construction is equipped with a separate high-voltage generation circuit part for obtaining a highly precise high-voltage load characteristic. For this reason, it is unavoidable that the switching circuit part is made up into a two-system construction. Here, separately providing the high-voltage generation circuit in such a way is surely very advantageous in terms of the characteristics. However, resultantly, there results the drawback that the circuit construction becomes complex and the problem that the loss of electric power of the switching circuit part increases.
Also, this switching power source apparatus, as a second problem, is equipped, in its power source circuit part, with an insulated type converter transformer for insulating it from the grounding earth. And it is also equipped, in its high-voltage generation circuit part, with a non-insulated type high-voltage transformer such as a flyback transformer. For this reason, the output converter transformer cannot but be made up into a construction of its being doubly equipped. Resultantly, in a construction wherein a high-level output voltage is taken out using switching means for performing the switching operation with the rectified/smoothed voltage obtained from a commercially available alternating power source being used as the operating power source, the direct current to direct current conversion efficiency becomes inferior. Resultantly, this apparatus has a point of problem in achieving the saving of the power.